Systems and methods for photovoltaic string protection

1. A circuit for protecting a photovoltaic string, comprising:

a bypass switch connected in parallel to the photovoltaic string;

a hot spot protection switch connected in series with the photovoltaic string;

a first control signal to control opening and closing of the bypass switch, where the bypass switch is normally open, and the first control signal closes the bypass switch upon detection of a determined condition at a photovoltaic cell while the hot spot protection switch is closed to allow current to flow through the photovoltaic string and to bypass the photovoltaic cell to halt and protect against an arc fault; and

a second control signal to control opening and closing of the hot spot protection switch, where the hot spot protection switch is normally closed, and the second control signal opens the hot spot protection switch after a delay after the bypass switch closes to prevent current flowing from the photovoltaic string from causing hot spotting to the photovoltaic cell.

2. The circuit of claim 1 , further comprising a converter bypass switch connected between a power bus and the photovoltaic string, where a converter connected with the photovoltaic string is bypassed when the converter bypass switch is closed.

3. The circuit of claim 1 , further comprising a differential power processing (DPP) unit bypass switch connected between converter and photovoltaic string units, where one of the converter and photovoltaic string units is bypassed when the unit bypass switch is closed.

4. The circuit of claim 1 , where the hot spot condition is detected when a photovoltaic positive voltage goes negative.

5. The circuit of claim 1 , where the hot spot condition is detected when a change in capacitance is detected.

6. The circuit of claim 5 , where the capacitance comprises photovoltaic (PV) junction capacitance.

7. The circuit of claim 1 , where the hot spot condition is detected when a change in impedance is detected.

8. The circuit of claim 7 , where the change is impedance comprises a change in magnitude or phase for a determined frequency range.

9. The circuit of claim 7 , where a magnitude of a perturbation waveform in current and voltage measurements of the photovoltaic string is used to measure the impedance.

10. The circuit of claim 1 , where a hot spot condition is detected when at least one of a capacitance increases and an impedance magnitude decreases over determined frequencies for a photovoltaic string.

11. A circuit for protecting a photovoltaic string, comprising:

a hot spot detection circuit configured to detect at least one of a change in an impedance magnitude and capacitance of the photovoltaic string;

a bypass switch connected with the hot spot detection circuit, the bypass switch also connected in parallel with the photovoltaic string, the bypass switch configured to close when the hot spot detection circuit detects a decrease in the impedance magnitude or an increase in capacitance to allow current to flow through the photovoltaic string and bypass a photovoltaic cell to halt and protect against an arc fault; and

a hot spot protection switch connected in series with the photovoltaic string, the hot spot protection switch configured to open when the hot spot detection circuit detects a decrease in the impedance magnitude or an increase in capacitance, where the hot spot protection switch opens after a delay after the bypass switch closes to prevent current flowing from the photovoltaic string from causing hot spotting to the photovoltaic cell.

12. The circuit of claim 11 , further comprising a converter bypass switch connected between a power bus and the photovoltaic string, where a converter connected with the photovoltaic string is bypassed when the hot spot detection circuit detects a decrease in the impedance magnitude or an increase in capacitance.

13. The circuit of claim 11 , further comprising a unit bypass switch connected between converter and photovoltaic string units, where one of the converter and photovoltaic string units is bypassed when the hot spot detection circuit detects a decrease in the impedance magnitude or an increase in capacitance.

14. The circuit of claim 11 , where the capacitance comprises photovoltaic (PV) junction capacitance.

15. The circuit of claim 11 , where the bypass switch is further configured to close when the hot spot detection circuit detects a change in impedance phase for a determined frequency range.

16. A method for protecting a photovoltaic string, comprising:

controlling with a first control signal opening and closing of a bypass switch connected in parallel with the photovoltaic string, to allow current to flow through the photovoltaic string and bypass a photovoltaic cell to halt and protect against an arc fault;

controlling with a second control signal opening and closing of a hot spot protection switch, to prevent current flowing from the photovoltaic string from causing hot spotting to the photovoltaic cell; and

detecting a hot spot condition, where the first control signal closes the bypass switch and after a delay after the bypass switch is closed the second control signal opens the hot spot protection switch when the hot spot condition is detected.

17. The method of claim 16 , further comprising controlling with a third control signal a converter bypass switch connected between a power bus and the photovoltaic string, and bypassing a converter connected with the photovoltaic string when the hot spot condition is detected.

18. The method of claim 16 , further comprising controlling with a fourth control signal a unit bypass switch connected between converter and photovoltaic string units, and bypassing one of the converter and photovoltaic string units when the hot spot condition is detected.

19. The method of claim 16 , where a hot spot condition is detected when at least one of a capacitance increases and an impedance magnitude decreased over determined frequencies for a voltage operating range of the photovoltaic string.